Major standards established by IEEE 802.16 working group include IEEE 802.16-2004 called fixed WiMAX and IEEE 802.16e-2005 (referred to as 16e hereinafter) called mobile WiMAX. IFFF 802.16e-2005 was finally approved as a standard by the IEEE in December 2005. Standards that are the basis of mobile WiMAX of the present version are IEEE 802.16-2004, IEEE 802.16e-2005 (this document includes corrigenda of IEEE 802.16-2004), and IEEE 802.16-2004/corrigenda2/D8. Standardization of IEEE 802.16m (referred to as 16m hereinafter) for mobile WiMAX of the next version is in progress in TGm under the IEEE 802.16 working group.
A process through which a mobile station adjusts transmission parameters (a frequency offset, a time offset, and transmit power) for uplink communication with a base station during an initial network registration procedure is referred to as initial ranging. After a network registration procedure, the mobile station performs periodic ranging in order to continue uplink communication with the base station. Ranging procedures include handover ranging for simplifying a handover procedure of a mobile station and bandwidth-request ranging performed during a procedure through which a mobile station requests an uplink band upon generation of data to be transmitted.
In a broadband wireless access system, a CDMA code (or ranging preamble) set that can be used for ranging according to ranging type and a region in which a CDMA code is transmitted are allocated through a channel (e.g. UL-MAP) through which system information is broadcast by a network. Accordingly, if a specific mobile station performs handover ranging, the mobile station needs to request handover ranging by selecting a specific code from CDMA codes for handover ranging and transmitting the selected code through initial ranging and handover ranging regions to the network. According to this scheme, the network can distinguish the ranging type through the received CDMA code and the region in which the CDMA code is transmitted.
More specifically, an IEEE 802.16m advanced mobile station (AMS) acquires downlink synchronization with a base station that is a target of ranging and receives uplink transmission parameters in order to perform initial ranging. The AMS selects a ranging slot (or ranging opportunity) according to random backoff, randomly selects an initial ranging code and transmits the initial ranging code in the selected ranging slot. Upon reception of the ranging code, the base station transmits a ranging acknowledgment (AAI_RNG-ACK) message to the AMS. The AAI_RNG-ACK message may include the ranging code transmitted by the AMS in a predetermined frame, code reception status and a physical correction value according to reception status.
Specifically, upon reception of ranging codes from AMSs, the base station transmits allocation information of the AAI_RNG-ACK message as a response to the ranging codes to the AMSs through a broadcast assignment A-MAP information element (IE) using a broadcast masking code. When the AAI_RNG-ACK message is transmitted to a specific AMS as an unsolicited signal, the allocation information is transmitted to the AMS through a downlink basic assignment A-MAP IE using a unicast station identifier (STID). In a specific case, for example, if all the ranging codes are successfully received, transmission of the AAI_RNG-ACK message may be omitted.
If there is no ranging code successfully decoded in the ranging slot selected by the AMS or there is no response to an initial ranging attempt, which corresponds to the AAI_RNG-ACK message, when the AMS receives the AAI_RNG-ACK message and checks a ranging acknowledgement bitmap (RNG-ACK bitmap) of the AAI_RNG-ACK message, the AMS can determine that the initial ranging attempt has failed. Accordingly, the AMS restarts the initial ranging procedure.
When initial ranging is successfully performed, the base station may provide the following three types of responses to the AMS through the AAI_RNG-ACK message. First, when a ranging status is “continue”, a physical correction value is included in the corresponding message. Second, when the ranging status is “success”, the base station allocates an uplink resource for ranging request message transmission to the AMS. Here, the physical correction value may be included. Finally, when the ranging status is “abort”, the base station may request the AMS to interrupt the ranging procedure for a predetermined time.
The AMS continues the ranging procedure using the physical correction value included in the AAI_RNG-ACK message when the ranging status is “continue”. When the ranging status is “success”, the AMS is allocated the uplink resource by the base station and transmits an AAI_RNG-REQ message to the base station through the uplink resource.
The AAI_RNG-ACK message is a response to handover/periodic ranging as well as initial ranging, and the base station transmits the AAI_RNG-ACK message within a predefined time (i.e. a duration: transmission offset).
Upon reception of “continue” status and physical correction information through the AAI_RNG-ACK message, the AMS adjusts timing, power, frequency, etc. according to correction information, in general. Then, the AMS transmits a CDMA ranging code randomly selected from an initial ranging sequence domain to the base station. In this procedure, if another AMS transmits the same code as that transmitted by the AMS at the same time, the base station recognizes collision and does not perform the next process (e.g. a process of transmitting “continue” status or allocating an uplink resource for AAI_RNG-REQ message transmission).
In this case, the corresponding AMS may re-perform the initial ranging code transmission procedure or move to a different base station. This is because an operation for re-checking some parameters (i.e. correcting physical parameters according to “continue” status) is performed in a contention-based environment. This is generated in handover ranging or periodic ranging as well as in the initial ranging.
Accordingly, it is necessary to seek for a method for avoiding collision due to re-attempt in a post-ranging procedure of the AMS that receives “continue” status.
Furthermore, when the AMS re-transmits a ranging code upon receiving “continue” status, the AMS waits for a time corresponding to a value determined by a backoff algorithm and then re-transmits the ranging code. Even when the ranging code transmitted by the AMS is a dedicated ranging code, a wait time according to the backoff algorithm is not changed. Because the backoff algorithm is used as a method for minimizing code collision, application of the backoff algorithm to an AMS using a dedicated ranging code causes unnecessary delay.